Projection Objective For Microlithography

ABSTRACT

A projection objective for imaging a pattern arranged in an object surface of the projection objective into an image surface of the projection objective with a demagnified imaging scale has a plurality of optical elements which arc arranged along an optical axis of the projection objective and are configured in such a way that a defined image field curvature of the projection objective is set in such a way that an object surface that is curved convexly with respect to the projection objective can be imaged into a planar image surface. What can be achieved given a suitable setting of the object surface curvature is that a gravitation-dictated bending of a mask does not have a disturbing effect on the imaging quality.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a projection objective for imaging a pattern ofa mask arranged in an object surface of the projection objective into animage field arranged in the image surface of the projection objectivewith a demagnifying imaging scale, and to a microlithography projectionexposure apparatus having such a projection objective.

2. Description of the Related Prior Art

Photolithographic projection objectives with a demagnifying imagingscale (reduction objectives) have been used for several decades for thephotolithographic fabrication of semiconductor components and otherfinely patterned devices. They serve for projecting the pattern of amask, e.g. of a photomask or of a reticle, onto an article coated with alight-sensitive layer with very high resolution on a demagnifying scale.

Conventional projection systems are designed to image a planar mask ontoa planar image field. Accordingly, measures for correcting the imagefield curvature (Petzval correction) are provided in the projectionobjectives. The article “New lenses for microlithography” by E. Glatzelin: SPIE Vol. 237 (1980), pp. 310-320, describes known measures forleveling the image field.

The patent U.S. Pat. No. 5,052,763 describes a catadioptric projectionobjective with intermediate image, wherein the image of the object field(intermediate image) generated by a first, catadioptric subsystem isimaged into the image plane with the aid of a second, refractivesubsystem. In order to be able to image a planar object into a planarimage surface, the Petzval sum of the system is obtained by compensationof the image field curvature generated by the first subsystem by meansof the second subsystem, a curved intermediate image surface beinggenerated.

The patent U.S. Pat. No. 4,812,028 describes a microlithographyprojection objective having aplanatic refracting surfaces, non-aplanaticrefracting surfaces and reflective surfaces. The Petzval sums of theaplanatic refracting surfaces and of the remaining surfaces arecorrected independently of one another.

For projection lithography onto curved substrates, the U.S. Pat. No.6,461,908 B1 proposes using a curved mask whose form is identical to theform of the curved substrate. The curved mask is produced in a contactmethod. Curvature-conforming imaging of the curved mask onto the curvedsubstrate requires projection objectives with a substantial correctionof the image field curvature.

The U.S. Pat. No. 5,527,139 discloses a purely reflective reductionobjective for extreme ultraviolet radiation (EUV), wherein the objectsurface and/or the image surface are curved concavely with respect tothe projection objective.

The patent application US 2003/0133087 A1 describes a method by whichimaging errors that may result on account of thegravitational-force-dictated curvature of a reticle are intended to beprevented. This involves taking account of the fact that agravitational-force-dictated mask bending can lead to a distortion ofthe pattern situated on the mask (pattern stretching), so that errorssimilar to a distortion error result in the image of the pattern. Inorder to correct these errors, an optical correction element designedfor a distortion correction is introduced between the object plane andthe projection objective.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a projection objective bymeans of which adverse influences of the force of gravity on the imagingquality can be avoided.

This object is achieved, in accordance with a formulation of theinvention, by means of a projection objective for imaging a patternarranged in an object surface of the projection objective into an imagesurface of the projection objective with a demagnified imaging scale,the projection objective having a plurality of optical elements whichare arranged along an optical axis of the projection objective and areconfigured in such a way that a defined finite image field curvature ofthe projection objective is set in such a way that an object surfacethat is curved convexly with respect to the projection objective can beimaged into a planar image surface.

The projection objective is thus distinguished by the fact that apredetermined, finite value is provided for the image field curvature inorder intentionally to enable a non-curvature-conforming orcurvature-altering imaging between the curved object surface and theoptically conjugate, planar image surface or image plane with respectthereto.

As is known, the image field curvature dependent only on the radii ofthe refracting surfaces and the refractive indices of the opticalcomponents, with astigmatism eliminated and correction of the remainingaberrations, leads to the punctiform imaging of an object plane that isorthogonal with respect to the optical axis onto a curved image surface,the peak curvature of which is referred to as the Petzval curvature. ThePetzval curvature is proportional to the Petzval sum 1/R_(P), thereciprocal of which is the Petzval radius R_(P).

Conventional systems are set to a value of the Petzval sum as close aspossible to 0, so that the Petzval radius R_(P) should be infinite. ThePetzval correction, i.e. the correction of the image field curvature, istypically performed such that the magnitude of the Petzval sum is smallin relation to the depth of focus DOF, e.g. in accordance with|R_(P)|<0.1 DOF. As is known, the depth of focus is proportional toλ/NA², where λ is the operating wavelength and NA is the image-sidenumerical aperture of the projection objective. Projection objectivesaccording to the invention deviate substantially from this design goalwith regard to the correction of the image field curvature, so that abias of the image field curvature is created. In advantageousembodiments of projection objectives according to the invention, thefollowing holds true: |R_(P)|≧0.1 DOF.

In this case, the projection objective has an object surface that iscurved convexly with respect to the projection objective. Thus surfacecurvature is opposite to the “natural” surface curvature of a lenssystem with a positive refractive power overall and no or only weakPetzval correction and also a planar image surface. Accordingly, themeasures provided for correction of the image field curvature areintensified in comparison with a similar system with a planar objectsurface and planar image plane in the sense of an overcorrection of theimage shell.

A preferred embodiment with an object surface that is curved convexlywith respect to the projection objective is optimized toward takingaccount of a gravitation-dictated bending of the mask by means ofcorresponding measures for influencing the image field curvature. Theprojection objective is provided for utilization with an optical axisthat is oriented vertically in the region of the object surface, and theobject surface is curved in such a way that an effective object surfacecurvature in at least one direction perpendicular to the optical axisessentially corresponds to a gravitation-dictated mask bending of themask. As a result, the pattern of a mask that has bent on account ofgravity influences between bearing surfaces can on average be imagedsharply into a planar image surface, it being possible to avoid typicalerrors attributed to image field curvature. In the case of suchprojection objectives, the gravitation-dictated portion in the errorbudget is obviated, as a result of which stabler processes are madepossible.

This aspect of the invention is based on the consideration that, for agiven construction of a mask holder (reticle stage) and a predeterminedmask construction (with regard to size, thickness, material, etc.), a“fingerprint” of the gravitation-dictated bending of the mask existswhich is characteristic of these conditions and which is always presentin the same way as a systematic contribution and is translated into abending of the image surface with the square of the imaging scale in thecase of a Petzval-corrected system. The resultant contributions to theimage field curvature in the region of the substrate to be exposed mayperfectly well be of the order of magnitude of 10 to 50 nm, inparticular of 20-30 nm, in present-day systems. These contributions maybe tolerable as long as they are small relative to the depth of field ordepth of focus (DOF) of the projection objective. However, the latterdecreases drastically as the image-side numerical aperture NA increasesand the wavelength λ decreases, to be precise linearly with λ and withthe square of the numerical aperture. Therefore, the image errorsresulting from a gravitation-dictated mask bending may be disturbing,particularly in systems having high numerical apertures, for exampleNA>0.8 or >0.85. These problems are avoided in the embodiments mentioned

The gravitation-dictated bending of the mask is cylindrical to a firstapproximation in many mask holding systems and cannot be completelybiased in a rotationally symmetrical objective design. The situation isdifferent, however, in the case of projection objectives for waferscanners, in which the mask is moved relative to the projectionobjective in a scanning direction perpendicular to the optical axisduring the exposure operation (with synchronous movement of thesubstrate to be exposed). On account of the scanning operation, arotationally symmetrically curved object surface of the projectionobjective is translated into an effectively cylindrically curved objectsurface of the scanner system since only a certain excerpt from theobject surface is utilized for the imaging. A projection objectivedesigned for a curved object surface which is curved in such a way thata scanner-integrated object surface curvature corresponds to the surfacecurvature produced by gravitation-dictated reticle bending can sharplyimage a bent mask into a planar image surface over the entire imagefield.

In the case of projection objectives in scanner systems, this means thata cylindrical mask bending can be taken into account well throughsuitable correction of the image field curvature near the projectionobjective. In the case of projection objectives for stepper systems, inparticular, in which the mask and the article to be exposed arestationary during the exposure, it may be advantageous if at least oneoptical element of the projection objective bears at least onenonrotationally symmetrical surface, for example a toric surface.

The projection objective may be corrected with regard to allfield-dependent image errors (e.g. distortion), with the exception ofthe image field curvature. Preferably, it is also corrected well withregard to field-independent image errors, such as spherical aberration,so that essentially only the image field curvature remains asuncorrected image error.

In the design of the projection objective, it must be taken intoconsideration that the image shell error that leads to an image fieldcurvature or to a non-curvature-conforming imaging produces a variationof the image position over the image field. In contrast thereto,distortion is a variation of the image size. Therefore, the two errorscan be fundamentally distinguished from one another. The image shell,represented e.g. by the Petzval curvature, is influenced by the sum ofall surface refractive powers, to be precise first and foremost by meansof the radii of the refracting surfaces and secondly by the refractiveindices. At least the lowest-order image field curvature cannot becorrected by aspheres. The distortion, by contrast, can be influenced bydeflecting the main imaging beam in the tangential direction. Thedistortion can be corrected at locations with large principal vayheights by means of suitable radii and thicknesses of lenses, but alsoby means of aspheres. In contrast to the correction of the image shell,the distortion cannot be influenced in the vicinity of the aperturediaphragm.

In order to produce a projection objective according to the invention,the following procedure is preferably adopted. Firstly, the projectionobjective is calculated in a conventional manner such that it issuitable for imaging a planar object surface into a planar image surfacethat is optically conjugate with respect thereto, with a correspondingimaging scale. A lens is then picked out in the case of which thesurface radius is altered at one of the lens surfaces so as to result inthe desired change in the Petzval sum or the image field curvature. Thesystem is subsequently tuned, for example by adapting air clearancesbetween the individual lenses in order to minimize alternations broughtabout by this modification in the case of other aberrations. In the caseof new construction of projection objectives, such a lens which leads toa desired overcorrection of the image shell may be provided from theoutset. If existing systems are intended to be optimized in accordancewith the invention, then it is possible to exchange a lens provided at asuitable location for a lens with a changed surface radius and then tocarry out the corresponding tuning.

Well-correctable projection objectives are possible in the context ofthe invention. There are embodiments which are designed as “dryobjectives”. Dry objectives are distinguished by the fact that they aredesigned for a gas-filled gap to be present during operation between theexit side of the projection objective and the coupling-in surface of anarticle to be exposed, for example a wafer, the gap width of said gaptypically being significantly greater than the operating wavelength. Inthe case of such systems, the numerical apertures that can be achievedare restricted to values of NA<1, since, when approaching the valueNA=1, total reflection conditions occur at the exit surface and preventillumination light from being coupled out from the exit surface. Inpreferred embodiments of dry systems, the image-side numerical apertureis NA>0.8, NA≧0.85 or NA≧0.9 also being possible.

Projection objectives designed as immersion objectives are also possiblein the context of the invention. In the case of immersion lithography,as is known, the resolution that can be achieved in an exposure processis improved by introducing an immersion medium having a high refractiveindex, for example an immersion liquid having a refractive indexn_(I)≧1.3 at the operating wavelength, into the space between the lastoptical element of the projection objective and the substrate.Projection objectives or imagings having an image-side numericalaperture NA>1.0 are possible as a result, preferably NA≧1.1 or NA≧1.2 orNA≧1.3 being possible.

The optical construction also permits a use for contactless near fieldprojection lithography. In this case, it is possible for sufficientlight energy to be coupled into the substrate to be exposed via agas-filled gap if a sufficiently small image-side operating distance iscomplied with on average over time. Said operating distance should beless than four times the operating wavelength used, in particular lessthan the operating wavelength. It is particularly favorable for theoperating distance to be less than half of the operating wavelength, forexample less than a third, a quarter or a fifth of the operatingwavelength. Given these short operating distances, an imaging in theoptical near field may be effected in the case of which evanescentfields that exist in direct proximity to the last optical surface of theimaging system are utilized for imaging.

The invention can be used in purely refractive projection objectives aswell as in catadioptric projection objectives.

The above and further features emerge not only from the claims but alsofrom the description and the drawings, in which case the individualfeatures may be realized in each case on their own or as a plurality inthe form of subcombinations in an embodiment of the invention and inother fields and may represent advantageous and intrinsicallyprotectable embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows, in an oblique perspective illustration, anexcerpt from a microlithography projection exposure apparatus with anembodiment of a projection objective according to the invention;

FIG. 2 shows a schematic illustration of the rotationally symmetrical,curved object surface of the projection objective in FIG. 1 with ascanner slot;

FIG. 3 shows a schematic illustration of the form of the effectiveobject surface of the projection objective which is produced as a resultof scanning movement;

FIG. 4 shows a measured bending of a standard reticle in a schematicillustration;

FIG. 5 shows a schematic illustration of a bent reticle for calculatingthe theoretically expected mask bending; and

FIG. 6 shows a schematic illustration for quantifying the image fieldcurvature.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically shows the essential component parts of amicrolithography projection exposure apparatus in the form of a waferscanner 1 provided for the production of large-scale integratedsemiconductor components by means of projection lithography. Theprojection exposure apparatus 1 comprises, as light source, an excimerlaser (not shown) having an operating wavelength of 193 nm, otheroperating wavelengths, for example 157 nm or 248 nm, also beingpossible. A downstream illumination system 3, of which only the lightexit region is shown, generates in its exit surface 4 a large, sharplydelimited illumination field that is illuminated very homogeneously andis adapted to the telecentric requirements of the downstream projectionobjective 5. The illumination system 3 has devices for selection of theillumination mode and, in the example, can be changed over betweenconventional illumination with a variable degree of coherence, annularfield illumination and dipole or quadrupole illumination.

In the direction of light propagation downstream of the illuminationsystem there is arranged a device 40 (reticle stage) for holding andmanipulating a mask (reticle) 6 such that the latter lies in the objectsurface 4 of the projection objective 5 and can be moved in a travelingdirection (scanning direction) 7 (y direction) with the aid of a scannerdrive 41 for scanning operation.

Downstream of the object surface 4, the curved form of which will beexplained in more detail with reference to FIG. 2, there follows at asuitable distance (object-side operating distance) the reductionobjective 5, which images an image of the mask, with a reduced scale of4:1, onto a wafer 10 coated with a photoresist layer. Other reductionscales, e.g. 5:1 or 10:1 or less, are likewise possible. The wafer 10serving as a light-sensitive substrate is arranged such that its planarsubstrate surface 11 with the photo-resist layer essentially coincideswith the planar image plane 12 (depicted in dashed fashion) of theprojection objective 5. The wafer is held by a device 50 (wafer stage)comprising a scanner drive 51 in order to move the wafer synchronouslywith the mask 6 parallel to the latter.

The projection objective 5 is incorporated into the wafer scanner suchthat its optical axis 13 is oriented vertically and thus parallel to theeffective direction g of the force of gravity. The mask mount 40 isdesigned such that, apart from the force of gravity, no imposed forceswhich might lead to a deformation of the mask 6 occur at the reticle 6placed on said mount. Outside the region through which the illuminationradiation is to radiate, the transmission mask 6 is mounted on suitablebearing surfaces (or support surfaces) which are at a constructionallypredetermined bearing distance from one another (cf. FIG. 5). Betweenthe bearing surfaces, the reticle 6 is freely suspended and is exposedto the force of gravity g, which causes a gravitation-dictated maskbending. Depending on the type of reticle and the bearing geometry, agravitation-dictated bending is established in this case which is alwayspresent in essentially the same way as a systematic contribution and, inthe case of conventional Petzval-corrected systems, would be convertedinto a bending of the image of the mask with the square of the imagingratio. Given a standard size of currently used quartz glass reticles of6 inches·6 inches given a typical thickness of 6.35 mm, typicalinstances of gravitation-dictated bending may be in the range of between300 and 400 nm depending on the bearing geometry. In the case of typicalconventional systems that are optimized for imaging a planar objectsurface into a planar image surface, this reticle bending, given animaging scale of 4:1, would lead to an image field curvature of theorder of magnitude of between 20 and 25 nm. This indication of the imagefield curvature relates to the maximum excursion s′ of the image fieldIF in the image field center (at the optical axis OA) in comparison withthe axial position of the image field at the edge of the image field, orto a deviation s′—measured in the axially parallel direction—of thecurved image field from a plane IM lying perpendicular to the opticalaxis at the image field edge (cf. FIG. 6). This image field curvaturebecomes more critical the smaller the available depth of focus DOF ofthe projection system. Although it is possible to obtain a goodcompromise between sagittal and tangential image shell with the aid ofmanipulators by shifting lenses or displacing them in some other way,this is always accompanied by induced astigmatism on account of thePetzval condition.

These problems are avoided in the case of the embodiment of theprojection objective 5 shown. The projection objective 5 is designed forimaging an object surface 4 that is curved convexly with respect to theprojection objective (FIG. 2) into a planar image plane 12. Thus, incontrast to conventional systems, the mutually optically conjugatesurfaces do not have the same curvature state or a correspondingcurvature state transformed by way of the imaging scale, rather acurvature-altering imaging process is provided. In this case, theprojection objective 5 is designed such that all image errors, with theexception of the image field curvature, are completely corrected withinnarrow tolerances. By contrast, the image field curvature is altered bythe projection objective 5 such that a reticle 6 bent with respect tothe projection objective can be sharply imaged onto a planar wafer overthe entire image surface.

The gravitation-dictated bending of the reticle is cylindrical to afirst approximation. A complete bias for compensation of this warpage isnot possible in a rotationally symmetrical objective design. It can beapproximated, however. The situation is different in the case of ascanner objective, that is to say a projection objective provided foruse in a wafer scanner. On account of the scanning operation running inthe y direction, a rotationally symmetrically curved object surface 4(FIG. 2) of the projection objective is translated into an effectivelycylindrical object surface 4′ of the scanner system (FIG. 3). Thiseffect results from the fact that only the slotted excerpt 10 which isdepicted centrally in FIG. 2 and corresponds to the illuminated scannerslot is used for imaging. A movement of the approximately cylindricallycurved region of the scanner slot in the y direction produces thecylindrically curved effective object surface 4′ in FIG. 3. Thecurvature thereof is adapted, by the means for influencing the imagefield curvature that are provided within the projection objective 5, tothe reticle geometry of the bent reticle such that the mask structure tobe imaged essentially coincides with the effective cylindrically curvedobject surface 4, which is optically conjugate with respect to the imageplane 12. The mask 6 bent in the direction of the projection objectivecan thereby be sharply imaged onto the planar wafer 10 over the entireimage field diameter. A projection objective designed for an objectsurface which is curved convexly with respect to the projectionobjective and which is curved in such a way that the scanner-integratedobject surface curvature corresponds to the gravitation-dictated reticlebending will accordingly on average sharply image a bent reticle into aplanar image shell. As a result, the gravitation-dictated portion in thefocus budget is obviated and a stabler exposure process is possible.

Taking account of the reticle bending in the design of the projectionobjective can also be applied, in principle, to stepper systems. In thiscase, it is advantageous to generate the intervention in the image fieldcurvature with the aid of nonrotationally symmetrical, for example,toric, surfaces which may be applied on one or more lenses. Suitableaspheric forms are dependent on the bearing geometry of the reticle inthis case.

In order to explain the required order of magnitude of the image shellovercorrection of the projection objective 5, FIG. 4 firstly shows, in aperspective illustration, the measured, essentially cylindrically curvedprofile of the surface of a standard reticle (reticle size 6·6 inches,thickness 6.35 mm, material: quartz glass) bent in agravitation-dictated manner. FIG. 5 illustrates the conditions requiredfor deriving the suitable object surface curvature. The reticle 6 bearson two bearing surfaces 70 which are at a lateral bearing distance LAfrom one another. The bearing distance is greater than the object fielddiameter F, in order that the imaging is not disturbed by the bearings.The reticle has a thickness d and is composed of a mask material havingdensity ρ and modulus of elasticity E. Under the action of the force ofgravity g, a reticle bending s is established which, in thisillustration, is defined as the maximum excursion of the reticle in theg direction with regard to the reference plane 75 which is illustratedin dashed fashion and is defined by the bearing surfaces. A circle arc4″ which is defined by the bent reticle surface and represents theoptimum profile of the curved object surface for this bent reticlecorresponds to said bending s. The radius of the circle arc 4″corresponds to the object surface radius of curvature OFCR of the objectsurface in this direction running perpendicular to the optical axis.

Given this schematic geometry of the reticle mount, the theoreticallyexpected bending of the reticle results in accordance with:

$s = {\frac{\rho \; g}{4\; E} \cdot \frac{{LA}^{4}}{d^{2}} \cdot \frac{F^{2}}{{LA}^{2}}}$

An explanation will now be given in connection with FIG. 6 with regardto the image field curvature to which said bending leads on the imageside of the projection objective and what extent of the Petzvalcorrection is necessary for compensation of this effect. In thisrespect, FIG. 6 shows the image-side end of the projection objective,together with the region of the image field IF. The object-side reticlebending s is translated into an image-side image field curvature s′ withthe square of the imaging scale β in accordance with s′=β²·s. In thiscase, the image field curvature is parameterized by a deviations′—measured in the axially parallel direction—of the curved image fieldIF from a plane IM lying perpendicular to the optical axis OA at theedge of the image field IF. The edge of the image field is at a distanceh′ (image-side image height) from the optical axis OA. In the sectionalplane shown in FIG. 6, the image field IF is curved spherically to agood approximation, so that it lies on a circle arc having a radiusR_(P). This is the Petzval radius, for which the following holds true inaccordance with the circle equation for long radii to a goodapproximation: R_(P)=h′²/(2·s′). The Petzval sum 1/R_(P) of a systemwhich can image a mask having bending s into a planar image plane thusresults from the following equation:

${1/R_{P}} = {\frac{2 \cdot \beta^{2} \cdot s}{h^{\prime 2}}.}$

This estimation permits a corresponding bias of the image fieldcurvature to be provided in a projection objective in order to takeaccount of the effects of a gravitation-dictated reticle bending on theimaging quality.

A projection objective can be adapted by means of a fixedlypredetermined bias to the expected bending of typical reticles. It isalso possible to perform a dynamic adaptation by providing suitablemanipulators within the projection system in order, upon transition toother types of reticles, by way of example, to be able to perform achanged adaptation without reconstructing the projection objective.Suitable manipulators are, in particular, devices which bring aboutradii changes and/or refractive index changes within the projectionobjective. Refractive index changes may be brought about for example bymeans of pressure changes and/or temperature changes in the gas in lensinterspaces. Radii changes may be introduced by active opticalcomponents, e.g. by active mirrors. Heating or cooling a lens may leadto a change in refractive index and dimensioning of the lens andtherefore be utilized as a manipulator.

1-10. (canceled)
 11. A projection objective configured to image apattern arranged in an object surface of the projection objective intoan image surface of the projection objective with a demagnified imagingscale, comprising: a plurality of optical elements arranged along anoptical axis of the projection objective and configured such that theprojection objective has a defined image field curvature and images anobject surface that is curved convexly with respect to the projectionobjective into a planar image surface.
 12. The projection objective asclaimed in claim 11, wherein the object surface is curved such that aneffective object surface curvature in at least one directionperpendicular to the optical axis corresponds at least essentially to asurface curvature which results from a gravitation-dictated mask bendingof the mask.
 13. The projection objective as claimed in claim 12,wherein the effective object surface curvature corresponds to agravitation-dictated bending of the mask which is substantiallycylindrical.
 14. The projection objective as claimed in claim 11,wherein the projection objective is designed for use with a mask that ismoved in a scanning direction, and the object surface is curved suchthat an object surface curvature which is scanner-integrated in thescanning direction corresponds to the surface curvature caused bygravitation-dictated bending of the mask.
 15. The projection objectiveas claimed in claim 11, wherein at least one optical element of theprojection objective bears at least one nonrotationally symmetricalsurface.
 16. The projection objective as claimed in claim 15, whereinthe nonrotationally symmetrical surface is a toric surface designed forinfluencing the image field curvature.
 17. The projection objective asclaimed in claim 11, wherein the projection objective is corrected atleast essentially with regard to all field-dependent image errors otherthan the image field curvature.
 18. The projection objective as claimedin claim 11, wherein the projection objective is corrected at leastessentially with regard to all image errors other than the image fieldcurvature.
 19. The projection objective as claimed in claim 11, whereinthe projection objective has an image-side numerical aperture NA>0.8.20. The projection objective as claimed in claim 11, wherein |R_(P)|≧0.1DOF, where |R_(P)| is the magnitude of the Petzval sum and DOF is thedepth of focus of the projection objective.
 21. A projection objectiveconfigured to image a pattern arranged in an object surface of theprojection objective into an image surface of the projection objectivewith a demagnified imaging scale, comprising: a plurality of opticalelements arranged along an optical axis of the projection objective andconfigured such that the projection objective has a defined image fieldcurvature and images an object surface that is curved convexly withrespect to the projection objective into a planar image surface, whereinthe object surface is curved such that an effective object surfacecurvature in at least one direction perpendicular to the optical axiscorresponds at least essentially to a surface curvature which resultsfrom a gravitation-dictated mask bending of the mask, and wherein|R_(P)|≧0.1 DOF, where |R_(P)| is the magnitude of the Petzval sum andDOF is the depth of focus of the projection objective.
 22. Theprojection objective as claimed in claim 21, wherein the effectiveobject surface curvature corresponds to a gravitation-dictated bendingof the mask which is substantially cylindrical.